Prior art sound systems comprise one—or multi—channel transducer arrays to improve the quality of the transmission of target sound information. For example, transducer arrays such as the two loudspeaker boxes in a stereo set-up, may provide a listener with two similar sound signals. A binaural listener is able to localize a stereophonic sound source in-between the two loudspeakers if the ears receive the two signals more or less concurrently and with equal intensity as is disclosed in GB 394,325. The summing localization mechanism of the binaural ear-brain system then provides the listener with the impression that one or more virtual sources are present in-between the two adjacent physical speakers. This is disclosed in B. C. J. Moore, An Introduction to the Psychology of Hearing, 4th Ed., Academic, San Diego (1997) p. 232, 234. However, although especially with multi channel arrays, the transmitted sound is now perceived to have a better integrity of the target sound information than the sound of one speaker alone, such sound systems have the problem that the respective transducers of the array are localizable because the spatial-spectral character of the loudspeakers remains detectable, which affects the advantages of this technique negatively, as is disclosed in G. Theile, “Über die Lokalisation im Überlagerten Schallfeld”. Dissertation, Techn. Universität Berlin (1980)
All sound transducers which are impedance matched to the air, most efficiently interact with those sound waves that have wavelengths corresponding to the physical dimensions of the transducer structure. This effect influences the frequency response to vary with direction and causes resonant characteristics. The resulting sound wave fronts are modified by the so called baffle step. More information on the baffle step is found in the article of Andy Unruh, “Understanding Cabinet Edge Diffraction”, Unruh Acoustics, http://www.speakerdesign.net/understand.html. Reflection and diffraction of the waves according to the shape of the enclosure of the transducer element and by the shape of the transducer element itself, cause frequency dependent particle velocity gradients of the pressure differences which model the polar response pattern of the transducer structure. Relevant publications are Olson, H. F., “Direct Radiator Loudspeaker Enclosures”, JAES Vol. 17, No. 1, 1969 October, pp. 22-29; Joerg Panzer, “Far-field radiation from a source in a flat rigid baffle of finite size”, New Transducers Ltd, Huntingdon, U.K; and W. R. Woszcsyk, “The Increase of Transducer Directivity Using Diffractive Attachments”, J. Acoust. Soc. Am., Supplement 1, Vol. 84, 1988.
This shape related transfer function, further referred to here as the spatial-spectral contour of the transducer, is apparent in the sound waveforms reaching the ear-drum of the before mentioned listener from the speaker array. This spatial-spectral contour, which is also referred to as the spectral signature when an observer is involved, causes the localizability and affects the perceptual qualities of any resulting virtual sources embedded in the input signal, as disclosed in the publication: G. von Bekesy (1960), E. G. Wever (Editor), “Experiments in Hearing”, New York, N.Y., McGraw Hill. In the now following the spectral signature is used for both situations, it will be clear from the context what is actually meant.